This invention relates to an optical subscriber network having a delay measurement function, and to a delay measurement method. More particularly, the invention relates to a delay measurement method in an optical subscriber network having a station, a plurality of subscriber units and an optical transmission line through which optical signals are sent and received by the station and plurality of subscriber units.
A PON (Passive Optical Network) is one way of implementing an optical communications network that utilizes the broadband characteristic of optical fiber. An optical communications network of this kind is the focus of much attention as the network is ideal for providing multimedia services such as CATV and VOD (Video on Demand) to the home by way of optical fiber. A PON-implemented optical subscriber network generally has the configuration shown in FIG. 12. As shown in FIG. 12, the network comprises a station 1 such as a CATV station, subscribers (subscriber units) 21-2n, and an optical transmission line 3 which connects the station 1 with the homes of the subscribers using optical fibers 3a and a star coupler 3b to make possible the communication of main signals (video and audio signals as well as uplink/downlink control signals) in both directions.
Research utilizing wavelength multiplexing techniques to realize high-capacity transmission is being carried out in regard to optical subscriber networks of this kind, and transmission techniques using different wavelengths for downlink data (sent from station to subscriber) and uplink data (sent from subscriber to station) have been proposed. Techniques using wavelengths in the 1.55-xcexcm band for downlink data and in the 1.3-xcexcm band for uplink data are the focus of attention at the present time. Schemes for performing wavelength multiplexing in order to implement high-capacity transmission are being studied in regard to downlink data as well.
The same wavelength (e.g., a wavelength in the 1.3-xcexcm band) is used for all subscribers in regard to uplink data. Consequently, unless some measures are taken, uplink data transmitted by the subscribers will collide within the optical transmission line and it will not be possible to demodulate the data correctly at the station. To this end, the station specifies the timing of uplink data transmission for each subscriber and causes the uplink data to be transmitted at these timings so that collision of the uplink data is avoided.
However, the distances between the subscribers and the station are not the same but differ for each subscriber. As a consequence, delay times that develop between the subscribers and the station differ for every subscriber and collision of uplink data will occur unless consideration is given to the disparity in delay times. For this reason, the station measures the distance to each subscriber, namely the delay time, and decides the transmission timings of the uplink signals on the basis of the measured delays.
FIG. 13 is a diagram useful in describing delay measurement according to the prior art. The station 1 sends an optical signal S1i (i=1, 2, . . . ) for downlink delay measurement separately to each subscriber 2i. Upon receiving the signal S1i, the subscriber executes processing at a termination and sends an optical signal S2i for uplink delay measurement back to the station 1. The station 1 then measures the time from the moment the optical signal for downlink delay measurement was sent to the moment the optical signal for uplink delay measurement was received and adopts this time as the delay time.
However, it is required that this conventional method of delay measurement be performed separately for each subscriber. A problem which arises is that measurement of delay takes too much time.
Further, with delay measurement according to the prior art, it is required that the processing method used for delay measurement be changed depending upon whether the subscriber unit is operating or not. This makes measurement a complicated task.
Accordingly, an object of the present invention is to arrange it so that delays associated with a plurality of subscribers can be measured simultaneously in a short period of time.
Another object of the present invention is to make possible the measurement of delay time irrespective of whether a subscriber unit is operating, and to improve the accuracy and reliability of delay measurement.
A further object of the present invention is to arrange it so that delay can be measured upon specifying one or more subscribers at will.
Yet another object of the present invention is to arrange it so that a failure such as cable severance can be detected and the point of failure identified.
The foregoing objects are attained by providing an optical subscriber network having a station, a plurality of subscriber units and an optical transmission line through which optical signals are sent and received by the station and plurality of subscriber units, in which (1) wavelengths for delay time measurement are allocated to respective ones of the subscriber units; (2) delay-measurement optical signals having the aforesaid wavelengths are wavelength-multiplexed and sent to the optical transmission line from the station, the number of wavelength-multiplexed optical signals being equivalent to the number of subscribers; (3) the wavelength-multiplexed delay-measurement optical signals are distributed to each of the subscriber units via a star coupler in the optical transmission line; (4) each subscriber unit selects, by wavelength selection a delay-measurement optical signal having the wavelength that has been allocated to it, and loops back the selected optical signal; (5) the star coupler of the optical transmission line combines the delay-measurement optical signals sent back from each of the subscriber units and sends the combined signals to the station; and (6) the station separates, according to each subscriber unit by wavelength selection, the delay-measurement optical signals sent back, and measures a phase difference or time difference between a transmitted delay-measurement optical signal and a received delay-measurement optical signal to thereby measure transmission distance or delay time to each subscriber unit. In other words, by changing a wavelength xcexi for delay time measurement for every subscriber unit, it is possible to measure transmission distances or delay times from a station to all subscriber units at one time. As a result, measurement time can be shortened and the measurement operation simplified.
Further, in accordance with the present invention, the foregoing embodiments are attained by providing an optical transmission apparatus, which is provided in a station, comprising (1) optical transmitters for transmitting delay-measurement optical signals having wavelengths allocated to respective ones of subscriber units; (2) wavelength multiplexing/demultiplexing means for wavelength-multiplexing and sending, to an optical transmission line, the delay-measurement optical signals output by the respective optical transmitters, the number of wavelength-multiplexed optical signals being equivalent to the number of subscribers, and for demultiplexing, according to each subscriber unit by wavelength selection, delay-measurement optical signals sent back from the subscriber units; (3) optical receivers for receiving delay-measurement optical signals having wavelengths allocated to respective ones of the subscriber units; and (4) means for instructing each optical transmitter to send a delay-measurement optical signal, monitoring receipt of a delay-measurement optical signal by each optical receiver and measuring, for each subscriber unit, time from transmission of a delay-measurement optical signal to reception of the delay-measurement optical signal as delay time. In accordance with this optical transmission apparatus on the side of the station, it is possible to measure transmission distances or delay times from the station to all subscriber units at one time. Moreover, by specifying one or more subscribers at will, the delays associated only with these subscribers can be measured.
Further, in accordance with the present invention, the foregoing embodiments are attained by providing an optical transmission apparatus, which is provided in a subscriber unit, comprising (1) an optical coupler connected to a transmission line; (2) wavelength selection means connected to the optical coupler for selecting a delay-measurement optical signal having a wavelength that has been allocated to it; (3) a fiber delay line for delaying, by an amount equivalent to signal length, a delay-measurement optical signal selected by the wavelength selection means; (4) an isolator connected to the fiber delay line for transmitting a delay-measurement optical signal in one direction only; and (5) means for inputting a delay-measurement optical signal from the isolator to the optical coupler. In accordance with this optical transmission apparatus, a delay-measurement optical signal can be extracted and looped back irrespective of whether a subscriber unit is operating. As a result, the optical transmission apparatus on the station side can measure delay without giving consideration to whether or not the subscriber unit is operating. Moreover, it is possible to improve the accuracy and reliability of delay measurement.
Further, the optical coupler in the optical transmission apparatus on the side of a subscriber can be eliminated by connecting the isolator of the optical transmission apparatus on the subscriber side to a star coupler provided in the optical transmission line.
Further, in response to non-arrival of delay-measurement optical signals from all subscribers even upon elapse of a fixed period of time following transmission of the delay-measurement optical signals, it is possible to determine (1) whether a fault has occurred in the optical transmission line between the station and the star coupler or (2), in regard to all subscribers, whether a fault has occurred in the optical transmission line between the star coupler and the subscriber units or in the fibers within the subscriber units.
Further, in case of non-arrival of a delay-measurement optical signal from a specific subscriber unit even upon elapse of a fixed period of time following transmission of the delay-measurement optical signal, it is possible to determine (1) whether a fault has occurred in the optical transmission line between the star coupler and the subscriber unit or (2) whether a fault has occurred in the fiber within the subscriber unit.
Further, by providing means for measuring delay via a main-signal channel within the station and within a subscriber unit, a delay-measurement optical signal can be sent from the station to the subscriber unit via the main-signal channel and precise identification of a fault location can be performed based upon the answer from the subscriber unit.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.